SEDIMENTOLOGY, PALEONTOLOGY, AND STRUCTURAL FRAMEWORK OF THE CENTRAL ARKOSE RIDGE FORMATION, TALKEETNA MOUNTAINS, ALASKA

New sedimentological, paleontological, and structural data from Paleocene-Eocene strata in the southern Talkeetna Mountains document fluvial-lacustrine deposition and contractile deformation in a remnant forearc basin. This study focuses on the central part of the 60-km-long outcrop belt of Arkose Ridge Formation (ARF).

Measured sections document siliciclastic and minor volcanic strata with preserved thicknesses >1700 m. Four lithofacies associations dominate the stratigraphy. Boulder conglomerate and minor sandstone occur in paleovalleys along a basal unconformity. Cross-stratified sandstone, conglomerate, and carbonaceous mudstone record anastomosing fluvial systems with bifurcating channels and vegetated floodplains. Laminated carbonaceous mudstone, shale, and tuff represent small floodplain lakes. Cross-stratified sandstone, heterolithic sandstone/mudstone, in situ coalified tree trunks, and minor lavas record tidally influenced fluvial-estuarine deposystems.

A field collection of compression and coalified plant fossils from the central ARF includes ~70 leaves, axes, and cones in a diverse assemblage of dicots, cupressaceous conifers, and Equisetites, consistent with a floodplain forest community. Preliminary analysis of the ARF flora's leaf physiognomy and community composition when combined with broadly coeval assemblages (Chickaloon Fm.) exposed 5-15 km south of the study area indicate warmer P-E paleotemperatures than at present.

Geologic mapping demonstrates that boulder conglomerate with locally derived clasts was deposited along a high-relief surface. This relationship implies latest Cretaceous-Paleocene erosion of Cretaceous marine forearc strata. South-verging reverse faults and associated folds deform ARF strata. The structures together with age data from the ARF indicate post-Paleocene contractile deformation.

In summary, central ARF strata record fluvial-lacustrine deposition in a remnant forearc basin that was subsequently shortened by south-verging structures. The depositional and deformational history is consistent with tectonic models that infer Paleocene-Eocene spreading ridge subduction followed by Oligocene-Recent flat-slab subduction of the Yakutat microplate.